It is expected that in the near future all-optical networks, where opto-electric conversion takes place at or near end points, will prevail. Such networks employ a “connectionless” approach having similarities with the principles of IP packet routing. A demand for efficient optical packets and optical packet switching naturally comes into scene. WDM networks have already provided important benefits of increased available bandwidth for point to point optical links. However, the processing capabilities of electronic switches and routers are expected to impose serious bottlenecks on future optical networks. Improved optical switching technology is the key issue for increasing the potential for implementing optical packet switched networks. Optical packet switching provides greater flexibility and easier management for the network, because data remains in the optical domain essentially from source to destination. Therefore, not only delays due to opto-electronic conversion and electronic processing are avoided at switching nodes, but also a complete transparency to data and format is attained. Photonic switching, not having to perform opto-electrical conversion of data content and discarding protocols and data format recognition, thus combines higher switching speed with greater effective bandwidths which are compatible with throughputs above Tb/s. Further issues still under investigation and development, include packet routing, flow control and contention resolution.
Overviews of recent advances in photonic packet switching may e.g. be found in “Advances in Photonic Packet Switching: an overview”, by S. Yao and B. Mukherjee in IEEE Communic. Mag., pp. 84-94, February 2000 and in “Photonic Packet Switches Architectures and Experimental implementations” by D. J. Blumenthal, P. R. Prucnal and J. R. Sauer in Proceed. IEEE 82 (11), 1650 (1994).
Several solutions already exist, based on header and payload architecture for optical packets. Examples are found in e.g. “Self-routing of Wavelength Packets using All-Optical Wavelength Shifter and QPSK Sub-carrier Routing Headers” by E. Park and A. Willner in IEEE Photon. Tech. Lett. 8 (7), 938 (1996) and “HORNET—A Packet-Switched WDM Network: Optical Packet Transmission and Recovery” by D. Wonglumson, I. M. White, S. M. Gemelos and L. G. Kazovsky in IEEE Photon. Tech. Lett. 11 (12), 1692 (1999). The solutions differ mainly on whether the header is chosen in time, frequency, code or wavelength domain, or possible combinations.
In the published international patent application WO 00/04668, optical packets with data payload and separate header at a sub-carrier preceding the payload are used. However, problems may occur in this solution. As the optical packet propagates along the network a variable delay between header and payload may arise, which makes the switching procedures more difficult to perform. Moreover the header specifically occupies a frequency band above the payload baseband in order to avoid interference. When sub-carrier frequencies are as high as 10 GHz, the header may become severely attenuated due to fiber dispersion after tens of kilometers. This problem is recognized by the authors.
In the U.S. Pat. No. 5,253,250, an alternative solution is achieved by multiplexing a sub-carrier modulation (SCM) signal together with the payload signal. This is advantageous because payload and header experience the same light path and the same delays. The header sub-carriers occupy frequency bands above the payload baseband. Also here, header attenuation may result if the frequencies are too high. Moreover, devices for header processing have to be adapted to these higher frequencies, which may call for expensive and complex solutions. Since the general trend for the data transfer rate, and thereby the payload baseband, is to increase, sub-carrier frequencies which are possible to use today may in a short future be situated close to or even within the payload baseband. An optical network based on certain selected sub-carrier frequencies would then have to be re-designed in order to cope with higher payload frequencies.
When many nodes are being traversed, a necessity for acknowledgement signal may be implemented as in the U.S. Pat. No. 5,612,806. Also here, frequencies above the presently used payload baseband are used.